In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. One common attribute imparted to tissue sheets through the use of chemical additives is softness. There are two types of softness that are typically imparted to tissue sheets through the use of chemical additives. The two types are bulk softness and topical or surface softness.
Bulk softness may be achieved by a chemical debonding agent. Such debonding agents are typically quaternary ammonium entities containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the agent to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds within the tissue sheet.
Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the tissue sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface.
A multi-layered tissue structure may be utilized to enhance the softness of the tissue sheet. In this embodiment, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the tissue product. The outer layers of such structures may be composed of the shorter hardwood fibers, which may or may not contain a chemical debonder.
The topical or surface softness of a tissue sheet, and ultimately the resulting tissue product, may be achieved by topically applying an emollient to the surface of the tissue sheet or tissue product. The term emollient as used herein refers to a treatment capable of making a tissue sheet less harsh or abrasive. One such emollient is polysiloxane. Polysiloxane treated tissues are described in U.S. Pat. No. 4,950,545, issued on Aug. 21, 1990 to Walter et al.; U.S. Pat. No. 5,227,242, issued on Jul. 13, 1993 to Walter et al.; U.S. Pat. No. 5,558,873, issued on Sep. 24, 1996 to Funk et al.; U.S. Pat. No. 6,054,020, issued on Apr. 25, 2000 to Goulet et al.; U.S. Pat. No. 6,231,719, issued on May 15, 2001 to Garvey et al.; and, U.S. Pat. No. 6,432,270, issued on Aug. 13, 2002 to Liu et al., which are incorporated by reference to the extent that they are non-contradictory herewith. A variety of substituted and non-substituted polysiloxanes may be used.
While polysiloxanes may provide improved softness in a tissue sheet and/or tissue products, there may be some drawbacks to their use. Polysiloxanes are also generally hydrophobic, that is, they tend to repel water. For many tissue applications, particularly sanitary bath tissue, this significantly reduces the utility of polysiloxanes to create softness in the tissue product. Tissue sheets and/or tissue products treated with polysiloxane tend to be less absorbent than tissue products not containing polysiloxane. An additional disadvantage to the use of polysiloxanes in tissue sheets and/or tissue products, particularly hydrophobic amino functional polysiloxanes is the effect of aging on hydrophobicity. Elevated temperatures and time can significantly increase the hydrophobicity of treated tissue sheets and/or tissue products and in cases such as bath tissue may render the tissue product unacceptable for a given application after a certain period of time or under certain environmental conditions.
It is known to add a wetting agent directly to a polysiloxane emulsion then topically apply the polysiloxane, wetting agent composition to the tissue sheet to mitigate the hydrophobicity caused by addition of the polysiloxane. While this perhaps reduces the overall hydrophobicity of the sheet, there are several issues associated with using the wetting agents. First, wetting agents are hydrophilic and are usually incompatible with the neat polysiloxane. As such, if the wetting agent and polysiloxane are applied in the same step, they must be applied as an emulsion. Addition as a neat polysiloxane fluid is precluded.
During the production of tissue sheets and tissue products, significant amounts of scrap material are accumulated. This waste product, also known as broke, is generated from products that do not fall within manufacturer's specifications or from excess paper remaining after the finished product is completed. Since broke is comprised essentially of 100% fibers, ability to recycle it in tissue products eliminates the inefficient disposal of a valuable source of papermaking fibers. This broke is typically repulped and added directly to the virgin fibers in the tissue making process. As the wetting agents are water soluble or water dispersible they are prone to loss during the broke repulping and tissue making processes and, hence, the finished tissue sheet containing the polysiloxane treated tissue broke may contain a level of unwanted hydrophobicity.
Polysiloxane wetting agents are also known. The polysiloxane wetting agents are highly substituted low molecular weight polysiloxanes that are water soluble. As they are of low molecular weight and high degree of substitution they do not contribute to the softness of the tissue sheet. As with other wetting agents, they are not retained by the fibers and will be lost in the broke repulping and tissue making processes. Another disadvantage to the use of wetting agents is the buildup of the unretained wetting agents in the tissue process water. As the wetting agents function by reducing surface tension their buildup will reduce the surface tension of the process water. This reduction in surface tension of the process water causes unwanted reduction of the dry strength of the tissue web.
High molecular weight hydrophilic polysiloxanes are known in the art, however, such hydrophilic polysiloxanes are typically more water soluble and hence when applied to a tissue sheet will tend to migrate more in the z-direction of the tissue sheet than the hydrophobic polysiloxanes. The hydrophilic polysiloxanes are highly modified, replacing n-alkyl groups on the polysiloxane backbone with polyether or similar hydrophilic groups. Hydrophilic polysiloxanes typically are also usually sold at a cost premium to the hydrophobic polysiloxanes. The hydrophobic portion of the polysiloxane, referred to as the polydialkylpolysiloxane portion, also tends to have a more significant impact on improving softness. Hence, the highly modified hydrophilic polysiloxanes also tend to be less effective at softening and more costly to use than hydrophobic polysiloxanes.
Hydrophobic polysiloxanes may be blended with the high molecular weight hydrophilic polysiloxanes and such a blend topically to a tissue sheet and/or a tissue product to help mitigate the hydrophobicity issues associated with use of hydrophobic polysiloxanes. While such a blend helps to control and mitigate issues associated with hydrophobicity of the hydrophobic polysiloxanes, the hydrophilic polysiloxanes tend to migrate significantly more in the z-direction of the pretreated tissue sheet than the more hydrophobic polysiloxanes. Over time the hydrophilic polysiloxanes may migrate away from the hydrophobic polysiloxanes and with aging the hydrophobicity of the pretreated tissue sheet and/or tissue product may increase significantly to the point where the pretreated tissue product may no longer be suited for its intended application.
Additionally, the hydrophilic polysiloxanes generally described in the art have no functional group to anchor themselves to pulp fibers. As a result, these polysiloxanes may be readily lost to process water in the event that the polysiloxane treated tissue sheet and/or tissue product is used as a source of broke for additional tissue making processes. A couple of issues may result from the loss of the hydrophilic polysiloxane in the broke repulping operation. First, the polysiloxane contamination of the process water may cause significant issues in various process equipment and operations. Second, as the hydrophobic polysiloxanes may be retained in the wet end of the tissue making process due to the presence of functional groups, such as primary or secondary amines, tissue sheets and/or tissue products made from the broke fibers may exhibit unacceptable hydrophobicity if too much broke is used.
Therefore, there is a need for polysiloxane treated tissue sheets and/or tissue products having high levels of polydialkylpolysiloxane that have improved hydrophilic properties while still providing for softness enhancement in the polysiloxane pretreated tissue sheets and/or tissue products where they are incorporated. There is a further need to have the pulp fibers retain their hydrophilicity when recycled or used in broke and to have the pulp fibers and tissue sheets and/or tissue products containing the pulp fibers exhibit good thermal and aging stability with regard to hydrophobicity.
There is an interest in creating polysiloxane pretreated tissue sheets and/or tissue products that have softness equivalent to softness created by hydrophobic polydialkylsiloxanes, yet have excellent hydrophilic properties even upon thermal aging. There is a further interest in creating such polysiloxane pretreated tissue sheets and/or tissue products in a cost effective manner. Additionally, there is an interest in creating hydrophilic polysiloxane pretreated tissue sheets and/or tissue products exhibiting good retention of the polydialkylsiloxane through the tissue making process while maintaining good hydrophilic properties to enable expanded use of the polysiloxane pretreated tissue sheets and/or tissue products as a source for recycle and broke.
It has now been discovered that the present invention of blending certain amino functional polyether polysiloxanes with hydrophobic polysiloxanes and, in particular, aminofunctional polydialkylsiloxanes for treatment of pulp fibers for use in tissue sheets and/or tissue products provide such treated tissue sheets and/or tissue products having improved softness, hydrophilicity and aging stability while having high levels of polydialkylsiloxane. Both the polyether polysiloxane and the hydrophobic polysiloxane are retained very well through the wet end of the tissue making process yet the hydrophilic properties of the tissue sheets and/or tissue products made with recycled pulp fibers that contained the polysiloxane composition demonstrate excellent hydrophilic properties.